The present invention relates to an optical tape capable of recording and reproduction of information by the irradiation of light. More particularly, the present invention relates to an optical tape which is remarkably improved in recording and reproducing characteristics for information to be recorded.
Hitherto, magnetic recording media have been popularly used as information-recording tape. However, request for improvement of information-recording density has enhanced recently, and as an answer to such request, a recording device utilizing an optical recording and reproducing system has been proposed.
For example, Japanese Patent Application Laid-Open (Kokai) No. 57-33447 (1982) discloses a tape obtained by coating a plastic base film with a Te--As--Se metal film so as to have about 1,000 .ANG. in thickness and selectively forming the micropores in the metallic film by laser beams, and a tape obtained by coating the plastic base film with a mixture of carbon and nitrocellulose.
Japanese Patent Application Laid-Open (Kokai) No. 62-132251 (1987) discloses an optical tape produced by depositing Te--Se alloys, Se--In--Sb alloys or Ag--Zn alloys on a polyimide base film by means of Re-magnetron sputtering, and further forming thereon a protective layer.
Also, in Japanese Patent Application Laid-Open (Kokai) No. 1-286130 (1989) is disclosed an optical tape in which a magneto-optical layer is formed on one side of a base film and an anti-reflective layer is formed on the opposite side of the base film.
Although various types of optical tape have been proposed as mentioned above, the base materials used for such optical tapes had their own merits and demerits and there has yet been available no optical tape which is favorably put to practical use.
Polyester film is suited for use as a substrate of optical tape, as this film has excellent chemical, mechanical and thermal properties and it is also relatively easy to obtain a polyester film with a flat and smooth surface. However, a biaxially oriented polyester film with a flat and smooth surface had the problem that it tends to suffer damages such as scratching in the manufacturing process, thereby giving a bad influence upon the optical recording characteristics of the optical tape made by using the said polyester film.
Further, the optical tape using as its base film a polyester film having a flat and smooth surface could not be put to practical use because of poor slipperiness in the polyester film manufacturing process and the optical recording layer-forming process or in the optical tape running system. Also, the optical tape using as its base film a polyester film with good slipperiness does not show good reproducing characteristics when recording or reproduction of information is conducted with an optical recording and reproducing system. This is due to the fact that when the surface roughness of the substrate is high, although the running properties of the tape are superior, the optical reproducing characteristics lower, while when the flatness and smoothness of the surface of the substrate is improved, although the optical reproducing characteristics are superior, the running of the tape is deteriorated.
As a result of endeavors for solving these problems, it has been found that by using a specific substrate wherein a center-line average roughness (Ra.sup.B) of the surface on one side thereof is not more than 0.005 .mu.m and the number of coarse protuberances having a height of not less than 0.10 .mu.m in the surface on the same side is not more than 10 protuberances/cm2, the obtained optical tape is also excellent in optical recording and reproducing characteristics. The present invention has been achieved on the basis of this finding.